Production of aluminum



Dec. 10, 1968 E. L. HEATON ETAL 3,415,724

PRODUCTION OF ALUMINUM 3 Sheets-Sheet 1 Filed Dec. 16, 1965 ATTORNEY Dec. 10, 1968 5.1.. HEATON ETAL 3,415,724

PRODUCTION OF ALUMINUM 7 Filed Dec. 16, 1965 3 Sheets-Sheet 2 igii INVENTORS 50M 4. #54704! 3M0 Mil-DO 70???? 1?- BY A TTORNE Y Dec. 10, 1968 HEATON ETAL 3,415,724

PRODUCTION OF ALUMINUM Filed Dec. 16, 1965 3 Sheets-Sheet 5 an N INVENTORS [WW/ V Z. 1954/? ATTORNEY United States Patent 3,415,724 PRODUCTION OF ALUMINUM Edwin L. Heaton, Alcoa, Tenn., and Waldo Porter, Jr., Pittsburgh, Pa., assignors to, Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 16, 1965, Ser. No. 514,260 Claims. (Cl.- 20467) The invention relates to improvements in that method of winning aluminum from ores, such as the Hall process and which essentially consistsof the electrolytic conversion of aluminum ion available for said emersion in a molten bath. During such conversion the molten bath containing aluminum ion is subjected to a low voltage direct current flowing from an anode to an ultimate cathode, from whence by suitable arrangement the current is collected. In its essentials this process has not changed since its first specific embodiment, the Hall process, became first used for commercially producing metallic aluminum. However, the specific methods and instrumentalities by which the process is carried out have changed as search has continued for improvements enabling lower production cost in terms of lesser power per unit of metal produced and of lesser labor and material cost.

As used herein, and in the appended claims, the term molten bath is used to generally indicate an inorganic compound fusion containing ,in solution aluminum ion available to be converted to metallic aluminum under the action of an imposed electrical current, and the term "molten fluoride bath is used to indicate that specie of molten bath where the said.fusion is essentially composed of inorganic fluoride compound.

In the practice of the Hall,process the producing unit is a cell or pot. A number 01; such cells electrically connected in series constitute a producing assembly, or potline, in which line the current flows downstream from one cell to the next. The usually rectangular cells are, in essence, steel containers lined with a conducting lining, such as carbon. Disposed in --this conducting lining are metallic current collectors which transmit the current received by the lining, which acts as the ultimate cell cathode, to suitable conductor buses through which the current flows to the anodes of the next cell. Within the lined cell is disposed a molten fluoride bath, usually of the cryolite type, in which the aluminum oxide is dissolved or contained. The current is brought to the fused fluoride bath by one or more conducting anodes, usually non-metallic and made of carbonaceous material, the ends of which are positioned to contact the bath. The metallic aluminum produced collectsa iin a molten pool or layer under the fused bath by reason of its somewhat higher specific gravity and, thus, the current flows from the anodes into the molten fluoride bath and thence through the molten layer of aluminum to the lining of the cell, on which lining the aluminum layer rests.

Generally speaking, improvement in production cost of metallic aluminum must reside in reduction in cost of labor, cell maintenance, or materials consumed per unit of metal produced. In addition to the aluminum ore, one

of the materials consumed is electric power, and said consumption constitutes a major item of expense in aluminum production. One avenue of approach to cost reduction in terms of labor, of maintenance, and of power consumed is to increase the size of the cell unit and the amount of electric current imposed on the cell unit. One limiting factor to cost reduction by such increase of the current imposed on the cell is the fact that the imposed current must be collected and fed through conductors to the anode of a succeeding cell. Many other factors such as plant building size, desirability of compact operation, and the like, virtually dictate that the conductors which carry the collected current downstream from the cathode of one ICC cell to the anode or anodes of another cell must pass relatively close to the walls of each electrolytic cell. Thus as the total current carried by such conductors increases the electromagnetic effects of the current flowing in these conductors is also increased, the result being localized areas of flux intensity in.the molten layer of aluminum contained in the cell which results, for well known reasons, in impairment of cell operation such as short circuiting, metal splashing or spitting caused by excessive metal movement, local erosions of cathode lining, and the like. Thus, to a degree, the amount of current which may be imposed on a single cell has, as a practical matter, been limited by practical aspects of cell or pot-line construction. Usually, and. in cells and pot lines of the type herein specifically described, the electromagnetic flux causing these difliculties is a vertical component which is the resultant of one or more flux fields, and such a vertical component is referred to herein and in the appended claims as vertical flux or vertical electromagnetic flux.

When in the description'of the invention, and in the appended claims, the terms downstream and upstream are used such terms refer .to flow of electric current and the position of things in respect of that flow.

The principal object ofthe invention is the provision of a method of current handling in pot-line operations by which the amount of current imposed upon a cell in said line may be substantially increased without a corresponding and onerous increase in the cell of electromagnetic effects of a type and location intolerable to good operating conditions. A further object of the invention is the provision of means for the practice of said method.

The method of this invention, the specific objects above stated, other useful objects, the advantages of the invention and a preferred mode of applying: the invention to effect its intended purposes and achieve its advantages are herein described with reference to the attached drawings in which:

FIG. 1 shows diagrammatically and in perspective part of an electrolytic cell such as may be used in a pot-line for the production of aluminum;

FIG. 2 is a cross section on the plane II-II of FIG. 1;

FIG. 3 is a top view, with certain auxiliaries omitted, of an electrolytic cell embodying, in a preferred form, the principles of the invention; and

FIG. 4 is a side view, in perspective of the device shown in FIG. 3.

In general the like parts illustrated in the FIGURES 1 through 4 are designated by like numerals. A pot-line usually consists of a series of aluminum production cells arranged sequentially in .the sense that the current flows from one to the next. Often the cells are rectangular or square in shape and are jarranged in the pot-line side-byside with their sides, or ends, parallel. The electric current necessary to the reduction is initially fed from a power source to the first cell in line and returned to the power source, or usefully led elsewhere, from the last cell of the line. The cells are connected in series, the power flowing downstream from the cathode of the upstream cell to the anode of a downstream cell. Flow is indicated in the drawings by arrows. In the form generally indicated in FIGURES l and 2 the cells 10 are defined by steel containers 13 which are lined with insulation 14. A conducting lining 15, which serves as the ultimate cathode and may be formed of carbonaceous material in the form of pre-baked carbon blocks, or of carbonaceous material tamped into the cell in proper place and depth and thereafter ba'ked is supported on the insulation. Disposed in the carbon lining are the collector bars 12, there usually being one set or row disposed in the upstream side of the lining 15, and another set disposed in the downstream side of the lining 15. In operation each cell contains a layer 16 of molten bath. During operation the aluminum ion supply in this bath is replenished in any way, such as from dispensing devices, not here shown in order to avoid complication of illustration. During operation the aluminum metal produced gathers in bottom of the cell by gravity and forms a layer such as 17 (FIG. 2) underlying the molten bath 1'6, and resting upon the carbon cathode lining 15. The carbon anodes 11 are supported by conventional fasteners, not shown, from the longitudinal anode bus bars 19, by means of the metallic anode rods 18, which are secured to metallic attachments embedded in each anode. As the anode material is consumed during the metal producing operation the anodes are lowered, either by hand or by mechanical devices, to maintain their ends within the bath 16 at the desired level.

The current for the electrolysis flows from anode buses 19 through anode rods 18 and anodes 11 through the molten bath 16, the molten aluminum layer 17 and, thence, through cathode to the collector bars 12, from whence it is collected by the buses 20 and 21. It will be noted that collector bars 12 need not be separated as shown, but may pass completely through the cell lining from one side to the other. Electrically the result is the same. The upstream bus 21 extends around the ends of the cell toward the downstream bus 20 to which it may be connected, as shown in FIG. 1. The anode buses 19 of each cell receive the current collected by the collector bars 12 of the previous cell through connections 22 rising from the bus system 20, 21 to the buses 19. These connections, not shown in FIGS. 1 and 2, usually rise over the ends or over the upstream side of the cell to form the connection between the elevated bus structures 19 and the bus runs 20, 21 'which are generally lower disposed around cells 10.

The thickness of the molten layers 16 and 17 is not great, each amounting to a few inches. Therefore any extraordinary movement or displacement of layer 17 may cause a short circuiting of the current from an anode, or anodes, 11 directly to the aluminum layer 17. Additionally, abrupt or unusual motion of layer 17 may cause metal therefrom to splash, to spit out of the cell, or to locally erode the cathode lining 15. All such effects are undesirable. It is important, therefore, to maintain the molten metal layer in the cell relatively quiet, and of a given cross-sectional elevation throughout the process, and this is desirable whether the carbon anodes be of the pre-baked variety, as shown in the drawings, or of the Soderberg, or self-baking, variety of anode which, because of its larger size, is customarily used one to a cell.

The electromagnetic flux effects caused by the flow of current through the anode buses 19, and the cathode buses 20 and 21, tend to cause metal disturbing flux paterns in the molten aluminum layer of the cell. The shape, intensity, and general effect of these flux effects is influenced, as is well known, by the distance of the buses from the molten aluminum layer, the total current flowing per unit of time in a bus, the effect of metal structures, such as cell superstructures, on the pattern, the position and number of the risers, or connections, by which the cathode bus current collected from a previous cell is brought to the anode buses of a downstream cell, and various similar factors. The effect of such disturbance may be less onerous in one specific cell design than in another and toleration of molten aluminum disturbance of any given degree may, to an extent, be largely a matter of over-all cell design and/or particular operating conditions.

Where a pot-line composed of cells connected in electrical series is in operation the cathode current of an upstream cell is collected and then fed to the anodes of a downstream cell. Such procedure usually results, as a practical matter, in collection of a substantial amount of the cells cathode current in a bus or buses parallel the upstream side of the cell which bus, or buses, then curves around the upstream corners of the cell, or one of said corners, and proceeds alongside the ends of the cell in order to convey said current to a delivery position in respect of the next cell downstream. As hereinabove mentioned considerations of building size, compact operation, and cost of construction usually preclude removal of these current carrying buses to such a distance from the cell as to minimize the intensity of the flux which the current causes in the molten aluminum layer and, consequently, a considerable flux effect is caused in the molten aluminum layer.

When reduction of cost of aluminum production is attempted by imposing larger total currents on a cell, the amount of cathode current collected on the upstream face of the cell, and thus flowing around its upstream corners and downstream past its ends is increased and, finally, reaches a point where the disturbances created in the molten aluminum layer are no longer tolerable und r the conditions and thus become a limiting factor on the size of cell which can be used. In most cell constructions these conditions occur at some point after the total current impressed on the cell exceeds 100,000 amperes. In cell constructions of the type illustrated using pre-baked anodes, currents upward of 100,000 amperes impressed on the cell create collected cathode current flow around upstream corners of the cell which tends to so increase the intensity of vertical flux induced in the peripheral portions of the molten aluminum layer adjacent said 601'- ners as to preclude satisfactory cell operation. While the point of tolerance to such increases in intensity will vary widely with specific cell design it is doubtful if any of the cells of the general designs indicated in the drawings will tolerate much more than an impressed current of 160,000 amperes where the current collected on the upstream side of the cell is brought around upstream cell corners in approximately equal amounts. Where, of course, all of the current so collected is brought around one upstream corner the intolerable effect may occur at a lower total current value.

The method of the present invention comprises flowing a portion of the current collected on the upstream side of the cell under the cell, thereby diminishing the amount of current which flows through a conductor bus around an upstream corner and past the ends of the cell. While the method is here illustrated as applied to a cell construction in connection with which it is preferably practiced, the method can be practiced with any type of cell in which a current capable of creating onerous electromagnetic effects is collected from the cathode on an upstream side of the cell and thence conducted around a corner formed by said upstream side and an adjacent side of the cell, which adjacent side would in a rectangular cell be a cell end, on its path downstream to the next cell. The portion of the collected cathode current which is to be sent directly downstream without flow around said corner is less than one-half of the total current collected from the upstream portion of the cell.

A description of the method of this invention as operated in connection with aluminum reducing cells of pr ferred construction will now be given with reference to FIGS. 3 and 4. As in cells shown in the other figures, superstructure concerned with adjustment of anode height and of feed of aluminum ore to the molten bath has been eliminated as not directly pertinent to the invention and to allow clarity of illustration. In FIGS. 3 and 4 is shown a particular cell, a number of which are adapted for series-connected side-by-side pot-line operations. The cell is contained in a steel container 13. A cathode lining 15 of pre-baked carbon blocks of proper shape and size is supported in said container on an insulation layer 14 interposed between lining 15 and the steel shell 13. The cell is supported as a whole on a firm surface by supports such as those indicated at 23. Two anode buses 19 pass above the cell and with each of said buses are associated eight anodes 11, the electrical connection being provided by the metallic anode bars 18. The anode buses 19 are connected to the downstream flowing current by four rising connections, or risers, 22 which rise over the side of the cell from a point where, at a lower level, a connection, indicated at 24, is established with buses or other conductors bearing current collected from the cathode of a cell upstream in the pot-line to the cell here specifically under discussion. Of course if the cell under discussion should be the first cell of the pot-line, the connections 24 would suitably be with the power sources from which thepot-line current originates.

The illustrated cell of FIGS. 3 and 4 has forty collector bars 12 located in the lining 15, twenty of which extend outwardly of the cell for connection on the upstream side thereof and twenty of which extend outwardly of the cell for connection onthe downstream side thereof. Each set of collector bars extends almost to, but not to, the longitudinal center line of'the cell lining. Each collector bar 12 is embedded in lining 15 in the manner generally indicated at FIG. 2.. The cell has two assembled buses 21 and two assembled buses 20. The assembled buses 21 consist of 10 leaves or layers. Each layer is individually connected to one collector bar 12. Each bus 21 has eight layers each individually connected to one of eight collector bars 12 on the upstream side of the cell and these eight layers carry the current from such collector bars around an upstream corner of the cell, downstream past the cell end, or side-adjacent said corner and around the downstream corner of the cell where two other layers of the assembled bus 21 are furnished, on the downstream side, each to connect on that side to one of the two collector bars 12 which are closest to said downstream corner. Each of these ten layer assembled buses 21 terminate, as shown, in a connection 24 to a riser 22 of the next cell downstream in the pot-line, the riser being that closest to the end of the cell past which eight layers of the bus 21 runs. The two buses are also assembled buses each having six layers and each of said layers being connected, as shown, to the collector bars 12 which are the third through eighth from the respective ends of the cell and which protrude for connection in the downstream side of the cell. These assembled buses 20 each connect through a connection 24 to a riser 22 of the next cell downstream in the pot-line, each one of the buses 20 connecting to one of the two risers located in the center portion of the next cell. The risers 22 here shown actually rise upward to connect the buses 20 and 21 to the anode bus 19 at a desired point. However, the term riser is also used as a term of art to designate connections which, in their course, do not rise from one level to another.

Thus it will be noted that the assembled buses 21 and the assembled buses 20 account for the connection of all of the forty collector bars 12 except for the four bars which protrude for connection near the center of the cells upstream side and the four bars which protrude for connection near the center of the cells downstream side.

Attention is now directed to buses 25 which parallel each other and run under the cell 10* from the upstream side of the cell to the downstream side where each of the buses 25 is finally connected, by connectors such as 26, to each of the connections 24 provided for the two risers 22 which are located approximately in the center section of the next downstream cell of the pot-line. These buses 25 passing under the cell 111 connect through bus leaves or layers 27 with the four centrally located collector bars 12 protruding from the upstream side of the cell and the four centrally located collector bars 12 protruding from the downstream side of the cell thereby to ultimately deliver through connections 27, and to each of the two centrally located risers 22 of the next cell downstream, the current collected from four collector bars, two from the upstream, and two from the downstream side of the cell.

If, as is contemplated in the operation of a cell of the type illustrated in FIGS. 3 and 4, each collector bar 12 accounts for approximately one-fortieth of the total current delivered from the cathode lining 15, then approximately one-fifth of the current collected from the twenty collector bars located in the upstream side of the cell '10 is sent directly to the next cell in line through the buses 25 passing under cell 10 and thus does not go around the upstream cell corners through the assembled buses 21. Using cells of the type illustrated we have found that the diversion of this one-fifth of the current collected from the collector bars located on the upstream side of the cell is sufiicient to reduce vertical flux in the molten aluminum layer to a tolerable point when as little as 200,000 amperes are impressed on the cell.

The methods of this invention are-not necessarily limited to a diversion of a portion of the current collected from the upstream side of the cathode lining of the cell in a path which is under the cell. A path intermediate the upstream corners of the cell which is either over or under the cell would sufiice. However, as those skilled in the art are aware, the top side of a pot-line composed of electrolytic cells is an area of great activity during operation and is occupied with superstructure furthering such activities as the adjustment of anodes, feeding of ore, such as aluminum oxide to the molten bath, attendant crane activity, and the like. Consequently, the passage of electrical conductors directly over the cell and down to the next cell is usually neither mechanically feasible nor desirable.

While we do not contemplate a method in which more than half of the total current collected from the cathode lining on the upstream side of the cell is diverted directly downstream without following a. path around the upstream corners of the cell and pastj the end of said cell, we do contemplate within such limits said diversion of any amount of said current which will, by its failure to pass around said corners, reduce effectively vertical flux in the peripheral portions of the molten aluminum layer which are adjacent said corners.

In describin'g the methods of the invention, the specific cell structure discussed is one which by long and exlected and passed to the anode of the next cell, and in employing said methods. However, it will be immediately apparent to those skilled in the art that the method may be employed with other specific cell structures and whenever the current imposed on a cell causes disturbances in the cell-contained molten aluminum requiring control of vertical flux patterns.

Having thus described the methods of the invention, we claim:

1. In that method of electrolytically producing aluminum in a cell-contained, anode-contacted, molten bath, in whichf 'method the produced alii minum is collected in a molten layer underlying said bath in said cell and resting upon a conducting lining of; said cell which lining serves aslcathode of the cell, andin' which method a number of said cells are arranged sequentially and electrically connected in series to form a pot-line in which the current impressed on the anode circuit of a cell and collected from the cathode of said cell is conveyed to the anode circuit of the adjacent cell where it is again collected and passed to the anode of the next cell, and in which method a portion of the current collected from a cell cathode is collected on the upstream side of the cell and passed around at least one upstream cell corner and past the adjacent cell end to the anode of the next cell in the pot-line, the improvement of diverting a part but less than half of the current collected on the upstream side of the cell and directing said diverted current towards the anode of a downstream cell in said line in a path having a portion intermediate the corners of the cell from which the current is collected whereby to effectively reduce electromagnetic flux effects induced by the upstream collected current at least in areas of the said molten aluminum layer which lie adjacent said upstream corner.

2. In that method of electrolytically producing aluminum in a cell-contained, anode-contacted, molten fluoride bath, in which method the produced aluminum is collected in a molten layer underlying said bath and resting upon the cathode lining of said cell, and in which method a number of said cells are arranged sequentially and electrically connected in series to form a pot-line in which the current impressed on the anode circuit of a cell is collected from the cathode of said cell and conveyed to the anode circuit of a downstream cell in said line, and in which method a portion of the current collected from a cell cathode is collected on the upstream side of said cell and is passed around the upstream corners of the cell in its travel to the anode circuit of a downstream cell of said line, the improvement of diverting a part but less than half of the current collected on the upstream side of the cell towards the anode circuit of said downstream cell in a path having a portion intermediate said upstream corners whereby to effectively reduce vertical electromagnetic flux effects induced in areas of said molten aluminum layer which lie adjacent said upstream corners.

3. The process set forth in claim 2 characterized by the further fact that the diverted current passes intermediate the upstream corners of the cell in a least one path located below said cell.

4. The process set forth in claim 2 characterized by the further fact that the diverted current is all passed below the cell and intermediate the upstream corners thereof.

5. The process set forth in claim 4 characterized by the further fact that the cells of said pot-line are approximately rectangular in shape and are arranged side-by-side.

6. The process set forth in claim 4 characterized by the fact that the pot-line is composed of a number of approximately rectangular cells arranged side-by-side and that the current collected from a cell is brought to the anode circuit of a downstream cell by risers which feed the anode circuit from the upstream side of said downstream cell.

7. In that method of electrolytically producing aluminum in a cell-contained, anode-contacted, molten fluoride bath, in which a pot-line is formed of a number of electrolytic cells arranged side-by-side sequentially and electrically connected in series so that the current from the cathode of a cell is lead upwardly over the side of a downstream cell to connect with the anode circuit of said downstream cell and in which a substantial portion of said collected current is collected from the upstream side of a cell and in its course to connect with the anode circuit of said downstream cell is led around the upstream corners of the cell from whence it was collected, the improvement of diverting a part but less than half of the current collected on the upstream side of said cell to the anode circuit of said downstream cell in at least two paths which pass under said cell and between the upstream corners thereof on their way to the anode circuit of the downstream cell.

8. In that method of electrolytically producing aluminum in which a molten layer of the produced aluminum is contained in the electrolytic producing cell in contact with the cathode of said cell and below an anode-contacted molten bath and in which a part of the electrical current impressed on the anode circuit of said cell is collected from the cathode thereof through connections located on one side of the cell and from said connections led around at least one cell corner partially formed by said side the improvement of diverting a part but less than half of said side-collected current to its destination by way of at least one conductor which passes intermediate the cell corners partially formed by said side and to and beyond the plane of the opposite side of said cell.

9. In that method of electrolytically producing aluminum from an ore thereof in which a molten layer of the produced aluminum is contained in the electrolytic producing cell in contact with the cathode of said cell and below an anode-contacted molten bath containing said ore, and in which a part of the electrical current impressed on the anode circuit of said cell is collected from the cathode thereof through connections located at one side of the cell and from said connections led around the cell corners partially formed by said side the improvement of diverting a part but less than half of said side-collected current to its destination by way of at least one conductor which passes intermediate the cell corners partially formed by said side and to and beyond the plane of the opposite side of said cell.

10. In that method of electrolytically producing aluminum from an ore thereof in which a molten layer of the produced aluminum is contained in the electrolytic producing cell in contact with the cathode of said cell and below an anode-contacted molten bath containing said ore, and in which a part of the electrical current impressed on the anode circuit of said cell is collected from the cathode thereof through connections located on one side of the cell and from said connections led around the cell corners partially formed by said side, the improvement of diverting a part but less than half of said side-collected current to its destination by way of at least one conductor which passes under said cell and intermediate said cell corners whereby to effectively reduce vertical electromagnetic flux induced in said molten aluminum layer.

References Cited UNITED STATES PATENTS 2,804,429 8/ 1957 Wleiigel. 2,824,057 2/ 1958 Thayer 20467 XR 2,872,404 2/ 1959 Obermann 204244 2,999,801 9/1961 Wleiigel 204244 HOWARD S. WILLIAMS, Primary Examiner.

D. R. VALENTINE, Assistant Examiner.

US. Cl. X.R. 204-243, 244 

1. IN THAT METHOD OF ELECTRYTICALLY PRODUCING ALUMINUM IN A CELL-CONTAINED, ANODE-CONTACTED, MOLTEN BATH, IN WHICH METHOD THE PRODUCED ALUMINYM IS COLLECTED IN A MOLTEN LAYER UNDERLYING SAID BATH IN SAID CELL AND RESTING UPON A CONDUCTING LINING OF SAID CELL WHICH LINING SERVES AS CATHODE OF THE CELL, AND IN WHICH METHOD A NUMBER OF SAID CELLS ARE ARRANGED SEQUENTIALLY AND ELECTRICALLY CONNECTED IN SERIES TO FORM A POT-LINE IN WHICH THE CURRENT IMPRESSED ON THE ANODE CIRCUIT OF A CELL AND COLLECTED FROM THE CATHODE OF SAID CELL IS CONVEYED TO THE ANODE CIRCUIT OF THE ADJACENT CELL WHERE IT IS AGAIN COLLECTED AND PASSED TO THE ANODE OF THE NEXT CELL, AND IN WHICH METHOD A PORTION OF THE CURRENT COLLECTED FROM A CELL CATHODE IS COLLECTED ON THE UPSTREAM SIDE OF THE CELL AND PASSED AROUND AT LEAST ONE UPSTREAM CELL CORNER AND PAST THE ADJACENT CELL END TO THE ANODE OF THE NEXT CELL IN THE POT-LINE, THE IMPROVEMENT OF DIVERTING A PART BUT LESS THAN HALF OF THE CURRENT COLLECTED ON THE UPSTREAM SIDE OF THE CELL AND DIRECTING SAID DIVERTED CURRENT TOWARDS THE ANODE OF A DOWNSTREAM CELL IN SAID LINE IN A PATH HAVING A PORTION INTERMEDIATE THE CORNERS OF THE CELL FROM WHICH THE CURRENT IS COLLECTED WHEREBY TO EFFECTIVELY REDUCE ELECTROMAGNETIC FLUX EFFECTS INDUCED BY THE UPSTREAM COLLECTED CURRENT AT LEAST IN AREAS OF THE SAID MOLTEN ALUMINUM LAYER WHICH LIE ADJACENT SAID UPSTREAM CORNER. 